-17.5% is the change from 80 to 66
Calculation steps:
[( 66 - 80 ) ÷ 80] x 100 = -17.5%
Formula
Percentage change = [(66 - 80) / 80] x 100
Subtract the new number by the original number
66 - 80 = -14
-14 ÷ 80 = -0.175
-0.175 x 100 = -17.5%
Example